1 Field of the Invention
The present invention relates to feed circuits for supplying current to a load resistor, and in particular to such a circuit with a means for protecting its output with unwanted voltages of an opposite polarity and which has a switching element which is substantially fully driven even given low output voltages.
2 Description of the Prior Art
A circuit is known from German patent application No. P 34 25 536.2, corresponding to U.S. Pat. No. 4,629,905, having a feed circuit for feeding a load resistor, wherein the feed circuit is conducted via an electronic switch controllable by a control means to a lead pair for the connection of a load circuit and wherein the electronic switch contains the control path of a first transistor in one of the feed current paths leading from the feed circuit to the lead pair, and a control terminal of the first transistor is conducted to such a control potential that the first transistor is conductive given an activated feed circuit and is inhibited given a deactivated feed circuit. The anode-cathode path of a thyristor is disposed parallel to the controlled path of the first transistor and the control electrode of the thyristor connected via a first resistor to the one principal electrode of the thyristor and, via a non-linear bipole in the manner of a Zener diode which is inhibited at voltages below a threshold voltage and is conductive at voltages above a threshold voltage, to the other principal electrode of the thyristor. The first resistor lies parallel to the control path of the thyristor. The load resistor can thereby be composed of one or more partial resistances. The first transistor can be a field effect transistor or a bipolar transistor.
The circuit arrangement described above two feed circuits which can be connected to a user with opposite polarity and has an apparatus for the optionally feeding the load resistor by means of the one or the other feed circuit. The output of the operative feed circuit is conducted to the load resistor via a switch device which contains an automatically engageable and disengageable electronic switch constructed with a transistor and a thyristor.
The electronic switch is transmissive given an activated feed circuit and is inhibited given a deactivated feed circuit. Since the feed circuit provided with the switch means and the further feed circuit are alternately activated or deactivated with oppositely polarized output voltage, the electronic switch prevents the output of a deactivated feed circuit from loading the output of an activated feed circuit. This is of particular significance given feed circuits wherein a diode is effective at the output, this diode being inhibited for the output voltage of the feed circuit but being poled in the conducting direction for the output voltage of the other feed circuit, and therefore practically constituting a short-circuit of the output voltage for the other feed circuit. Such a diode effective at the output may be a component part of a rectifier circuit of a converter or rectifier or may be a diode additionally inserted for the protection of the feed circuit.
When the switch means is controlled dependent on the output voltage of the operative feed circuit, difficulties may arise when operating conditions are possible wherein the feed circuit emits only a very low output voltage which does not fully drive the transistor or the electronic switch.
A circuit arrangement comprising two feed circuits which are connectable to a load resistor with opposite polarity is disclosed in German OS No. 32 16 497. In this known circuit arrangement, one feed circuit is composed of a constant current source. The further feed circuit is obtained by connecting a voltage limiter to the constant current source. For protection in the locating made, a reduction of the device voltage to, for example, a maximum of 60 V is undertaken. The two feed circuits are conducted via a common pair of feed current paths to a switch-over means which optionally connects the load resistor to the two feed current paths with the one or with the other polarity. Serving as switch-over means is a relay which enables automatic fault locating and is correspondingly driven.
The load resistor in this known circuit is a remote feed loop which feeds a plurality of series-connected users with constant DC. The remote feed loop is provided with shunt arms which respectively contain a diode polarized in the non-conducting direction for the remote feed voltage and a resistor disposed in series therewith. When a test voltage polarized opposite to the remote feed voltage is applied to the input of the remote feed loop and when the remote feed loop is interrupted at any location whatsoever, a current then flows in all shunt arms preceding the interrupt location. The sum current is measured at the feed location. An interrupted repeater field is allocated to every measured value.
The remote feed device is switched from constant current to constant voltage. This becomes more difficult the higher the remote field power is in comparison to the locating power. Further, the polarity reversal is relatively complicated given series operation of remote feed devices.
Another possibility for the optional feed of an electrical user with voltages having different polarities is employment of mechanical contacts with manual actuation.
Further, it is already known from W. S. Jahn, "Elektrisch Fernuberwachen und Fernbedienen", 1962, Richard Pflaum Verlag Munchen, Page 218 to use multiple electrical lines in electrical remote control wherein remote control signals having different polarities are employed.